a) Field of the Invention
The present invention is directed to a method and an arrangement for stabilizing the average emitted radiation output of a pulsed radiation source, particularly of an EUV source based on a gas discharge plasma (GDP) or a laser-produced plasma (LPP), an excimer laser, an F2 laser, or another pulsed radiation source. Radiation sources of this type are preferably applied in semiconductor lithography for producing electronic circuits.
b) Description of the Related Art
In photolithography processes in the semiconductor industry, a mask (with the structure to be imaged) is projected in a reduced manner on the semiconductor substrate (wafer) in a scanner.
Aside from the characteristics of the optical system (numerical aperture, depth of focus, imaging aberrations of the lenses or mirrors), the quality of the photolithography process is essentially determined by how accurately the radiated radiation dose can be maintained. According to V. Banine et al. (Proc. SPIE Vol. 3997 (2000) 126), this dose stability (dose accuracy) is determined by:                a) pulse quantization        b) pulse-to-pulse stability, and        c) spatial stability of the emitting volume.        
The pulse quantization a) is scanner-specific. The quantity of light pulses that can enter the moving slit during a scan can vary. A method for optimizing the pulse quantization is disclosed, for example, in U.S. Pat. No. 5,986,742 A. Quantities b) and c) are specific to the radiation sources themselves, Quantity c) is meaningful only for EUV sources based on detectable fluctuations in the radiation-emitting plasma.
In conventional present-day exposure processes, a mask which is illuminated by a special light source is imaged in a reduced manner on the wafer. Narrow-band excimer lasers with wavelengths of 248 nm and 193 nm are currently used as radiation sources for exposure. Further, systems based on F2 lasers at 157 nm are in development, and the trend toward even shorter wavelengths continues at the present time through plasma-based EUV radiation sources with wavelengths of around 13 nm.
Every radiation pulse produces a radiation spot on the wafer surface which appears slightly displaced on the wafer surface from one pulse to the next due to a synchronous movement of the mask and the wafer. For each exposure of an exposure area (die), generally corresponding to a circuit, that is carried out on the wafer, the radiation source emits a predefined sequence of radiation pulses (burst), some of which pulses contribute to the total exposure of the photosensitive layer at a determined location on the wafer surface according to the speed of the movement, the reduction of the imaging optics and the size of the exposure area.
As regards dose stability (at a determined location on the surface of the wafer), chip manufacturers set extremely high demands on pulse-to-pulse stability. This is expressed in the standard deviation v of the current pulse energy from its average value or from a target value (set energy) of the pulse energy. Narrow-band excimer lasers in DUV lithography and VUV lithography require a dose stability of σD<1.5%, while EUV lithography even requires a dose stability of σD<0.3%
These demands can only be met by means of an active regulation of the radiation output on a pulse-to-pulse basis. For this purpose, all of the previously known methods rely on a variation of the reference value of the individual pulse energy of the next pulse ES,i+1 ES,i+1=f(E0, Ei, Ei−1, Ei−2, . . . ES,i, ESi,−1, ES,i−2. . . , i+1, . . . ),   (1)where E0 is the required average individual pulse energy, Ei is the actual energy of the preceding pulses, ES,i is the respective reference value of the individual pulse energy of the preceding pulses, i+1 is the number of the next pulse within the burst, and f( . . . ) is a mathematical relationship which describes how the next reference value is to be obtained from the above-mentioned values—that is, the implementation of the regulating algorithm to be used (e.g., PID regulation, etc.).
A method of this kind is presented, e.g., in U.S. Pat. No. 5,608,492 which describes a pulse-to-pulse energy regulation based on a closed control loop of light emission, light pulse energy measurement, determination of the charge voltage of the driving laser, and renewed light emission. Accordingly, it corresponds to the regulating principle expressed by Equation (1).
In this connection, it is left to the experience of the respective design engineer to produce a suitable relationship gn( . . . ) between the reference value of the energy for the next pulse ES,i+1 and one or more suitable manipulated variables Sn,i+1 (e.g., the charge voltage in a gas discharge-operated radiation source):Sn,i+1=gn(ES,i+1)   (2)
Additional relationships known to the system can be entered in the function(s) gn( . . . ) Examples have been described in other publications.
For example, DE 102 09 161 A1 discloses a regulating method for a pulsed radiation source which is capable of constantly re-determining the proportionality factor of a proportional regulation from the previous measured values for pulse energy and manipulated variable (charge voltage) thereby making the regulation more accurate.
U.S. Pat. No. 6,005,879 A describes a specific construction for an excimer laser system. In this case, the regulation is optimized by an improved knowledge of the relationship between charge voltage and the effect on the pulse energy of the laser. Accordingly, this regulation also falls under the general relationship gn( . . . ).
Further, DE 102 19 805 A1 states that regulation can also be carried out over more than one manipulated variable Sn,i+1 (in this case, charge voltage and gas pressure).
Other methods have been described, e.g., in DE 102 44 105 B3, which are directed to optimizing the regulation algorithm during the first pulses of a burst in order to compensate for transient effects in the initial phase.
All of the methods mentioned above are based on a conscious change in the reference value for the next radiation pulse. The particular publications differ only in the manner in which the next energy value is determined or in how the radiation source is controlled in order to match the next light pulse as closely as possible to the pulse energy target value.
U.S. Pat. No. 6,456,363 B2 discloses a device containing a pulse light source for emitting light pulses with varying light quantities. The intensities of the light pulses are measured and the intensity transmitted to the target (mask) is controlled by means of an adjustable light reducing unit.
U.S. Pat. No. 6,496,247 B2 describes an improved arrangement for this purpose in which the intervals between the light pulses are additionally taken into account by first carrying out a measurement of pulse energy and then calculating how many pulses are required for the required dose on the exposure area, and determining the speed of the scan over the mask and wafer from the timing of the pulses. When the scan speed is fixed, the light intensity is advantageously adjusted by fine filtering in such a way that no change in the quantity of pulses is required. On the other hand, if the average light quantity decreases, the number of pulses can be increased by one in order to achieve the necessary dose.
A similar procedure is also described in U.S. Pat. No. 6,538,723 B2, wherein after measuring the radiated pulse energy and determining the pulse frequency of the preceding pulses for the next pulse, the pulse energy is adapted so that the scan speed can remain the same.
The three publications cited above have the disadvantage that in order to adjust the required dose and achieve the required dose accuracy at a given scan speed and with a predetermined quantity of pulses in the window function of the exposed area, the pulse energy can be explicitly adapted only from the average pulse energy determined in the wafer plane. While the frequency of the radiation pulses is taken into account in order to ensure the required dose depending on the available pulse energy, a constant selection must be carried out for the exposure process in order to regulate exclusively the pulse energy by means of sensitively adapted variable extinction filters for maintaining the dose accuracy.
However, in many cases, a suitable dependable relationship gn( . . . ) which allows a fast pulse-to-pulse regulation of the pulse energy cannot be found. In particular, control of the charge voltage for the gas discharge, which is used in all of the above-cited regulating methods, may fail to work, for example, when the imaging characteristics of the optical system also influence the radiation intensity measured in the wafer plane.
Accordingly, particularly with plasma-based radiation sources which, aside from the temporal energy fluctuations, also have spatial fluctuations of the plasma, the otherwise good correlation between charge voltage and pulse energy is sensitively disrupted, so that an increase in the energy generated in the plasma by a spatial change in the plasma (e.g., due to the etendue limiting of the optical system) is lost on the way to the wafer. In such a case, regulation of the pulse energy through control of the charge voltage is not successful.
Further, the relationship gn( . . . ) required for successful regulation may be subject to time fluctuations which are not known to the regulating algorithm. This necessarily leads to additional inaccuracy in the regulation.